Google Voice numbers are reported as landlines, there seems to be some way to verify that a number is actually mobile. Very likely that Mozilla's report as landline. Banks (capital one) have definitely balked at my google voice number.

What these burner numbers are great for are rewards programs. I sign up for every one I can with my GV number!

>Very likely that Mozilla's report as landline.

I doubt it. It looks like they're using twilio under the hood, and those are most definitely detected as VOIP numbers.

google voice numbers are identified as the voip number they are (bandwidth dot com etc) and virtually nothing takes them for identification

When I look up my Google Voice number, it shows up as VoIP (not quite landline) with the carrier being Grand Central - SVR. It seems likely that Mozilla's service will similarly show up as VoIP. Some places are filtering VoIP numbers from their SMS verification schemes, but most places will let you sign up for promotional texts from a VoIP number.

Where can you "look up" your number?

There was something recently around a game which was able to detect if a user's phone number was on a prepaid or postpaid plan. I had no idea carriers share this information with others.

It doesn't say anything about if a number is postpaid or prepaid.

What about account verification for Twitter/etc.?

I'd be very interested to know if they work for SMS-only applications. I guess it's only $5 to try and find out. If anyone has tried, please report back!